1. Field of the Invention
This invention relates to a method for producing a high tensile-high toughness steel plate for welded structures, having a tensile strength of not less than 50 Kg/mm.sup.2 by a direct quenching after rolling and tempering process.
2. Description of the Prior Art
It is known that a steel plate manufacturing process in which a rolled plate is directly quenched and tempered, which is generally called "direct quenching and tempering process" (hereinunder referred to as "DQT" process), can reduce manufacturing costs because it enables the omission of the reheating step in the manufacturing process of a conventional quench-and-tempered steel. In addition, since this process can generally obtain higher strength in comparison with a process in which a rolled plate is reheated before quenching (hereinunder referred to as "QT" process), it can reduce the amount of alloys to be added, whereby the cost for alloying elements is reduced and also toughness of weld joints as well as weldability is improved pronouncedly.
For example, the gist of the DQT process disclosed in Japanese Laid-Open Patent Publication Nos. 153730/1983 and 77527/1983 resides in the following:
(i) the compositions of a steel are intended for welded structures and are determined in consideration on the toughness of weld joints and cold cracking property in weld zone;
(ii) a quenching starting temperature is not less than Ar.sub.3 and, after rolling, both the recovery and recrystallization of the roll-worked structure are accelerated until the commencement of quenching, and/or steel chemistry is limited not to form such precipitates as to restrain the above-mentioned .gamma.-recrystallization beheviour.
(iii) after quenching, the plate is tempered by reheating it at a temperature of not higher than Ac.sub.1.
The conventional DQT process, however, is defective in that the low temperature toughness of DQT plates is inferior to that of a steel plate produced by the QT process. The conventional direct quenching (hereinunder referred to as "DQ") process is aimed at improving quench hardenability at the time of DQ by recovering and recrystallizing the roll-worked structure. For that purpose, for example, in the method disclosed in Japanese Post-Exam Patent Publication No. 3011/1983, a rolled material is subjected to hot rolling in a manner of a total rolling reduction of not less than 50% in the temperature range of not lower than the Ar.sub.3 transformation point, finishing the steel plate to a predetermined plate thickness. It, however, requires to hold rolled plates isothermally or to cool them slowly for 1 to 15 minutes in a temperature range between a temperature less than the Ac.sub.3 transformation point and the Ar.sub.3 transformation point, followed by quenching.
In such a DQ process, since the roll-worked structure is recovered and recrystallized in the isothermally holding stage or the cooling stage, the size of the quenched microstructure produced by the DQ process is approximately equivalent to the size of austenite grain existing immediately before quenching. Since the austenite grain size immediately before the DQ step is relatively coarse, it is scarecely possible to obtain adequate low temperature toughness after being subjected to the DQT process. On the other hand, in the prior method concerning on DQ process, it fails to obtain adequate quench hardenability, hence it is unable to get the aimed strength after DQT process, as far as the roll-worked structure is neither recovered nor recrystallized.